Dew Point in Compressed Air - Frequently Asked Questions

Dew Point in Compressed Air – Frequently Asked Questions

Vaisala has put together some FAQs into an application note that will help you understand what dew point is all about and how important it is to measure dew point in compressed air applications.The complete document can be downloaded on Vaisala’s website: Dew Point in Compressed Air FAQ’s

Ever wonder what dew point is exactly and the recommended best practices for measuring it in a compressed air system?

1. What is dew point?

Dew point temperature is a measure of how much water vapor there is in a gas. Water has the property of being able to exist as a liquid, solid, or gas under a wide range of conditions. To understand the behavior of water vapor, it is first useful to consider the general behavior of gases.

In any mixture of gases, the total pressure of the gas is the sum of the partial pressures of the component gases. This is Dalton’s law and it is represented as follows:

The quantity of any gas in a mixture can be expressed as a pressure. The major components of air are nitrogen, oxygen, and water vapor, so total atmospheric pressure is composed of the partial pressures of these three gases. While nitrogen and oxygen exist in stable concentrations, the concentration of water vapor is highly variable and must be measured to be determined.

The maximum partial pressure of water vapor is strictly a function of temperature. For example, at 20 °C (68 °F), the maximum partial pressure of water vapor is 23.5 mbar. The value of 23.5 mbar is said to be the “saturation vapor pressure” at 20 °C (68 °F). In a 20 °C (68 °F), “saturated” environment, the addition of more water vapor results in the formation of condensation. This condensation phenomenon can be exploited to measure water vapor content.

2. What is the difference between dew point and “pressure dew point?”

graphic-display

Instruments with graphical displays are useful for monitoring dew point over a longer period of time.

The term “pressure dew point” is encountered when measuring the dew point temperature of gases at pressures higher than atmospheric pressure. It refers to the dew point temperature of a gas under pressure. This is important because changing the pressure of a gas changes the dew point temperature of the gas.

3. What is the effect of pressure on dew point?

Increasing the pressure of a gas increases the dew point temperature of the gas. Consider an example of air at atmospheric pressure of 1013.3 mbar with a dew point
temperature of -10 °C (14 °F). From the table above, the partial pressure of water vapor (designated by the symbol “e”) is 2.8 mbar. If this air is compressed and the total pressure is doubled to 2026.6 mbar, then according to Dalton’s law, the partial pressure of water vapor, e, is also doubled to the value of 5.6 mbar. The dew point temperature corresponding to 5.6 mbar is approximately -1 °C (30 °F), so it is clear that increasing the pressure of the air has also increased the dew point temperature of the air. Conversely, expanding a compressed gas to atmospheric pressure decreases the partial pressures of all of the component gases, including water vapor, and therefore decreases the dew point temperature of the gas. The relationship of total pressure to the partial pressure of water vapor, e, can be expressed as follows:


By converting dew point temperature to the corresponding saturation vapor pressure, it is easy to calculate the effect of changing total pressure on the saturation vapor pressure. The new saturation vapor pressure value can then be converted back to the corresponding dew point temperature. These calculations can be done manually using tables, or performed by various kinds of software.

4. Why is knowledge of dew point in compressed air important?

The importance of dew point temperature in compressed air depends on the intended use of the air. In many cases dew point is not critical (portable compressors for pneumatic tools, gas station tire filling systems, etc.). In some cases, dew point is important only because the pipes that carry the air are exposed to freezing temperatures, where a high dew point could result in freezing and blockage of the pipes. In many modern factories, compressed air is used to operate a variety of equipment, some of which may malfunction if condensation forms on internal parts. Certain water sensitive processes (e.g. paint spraying) that require compressed air may have specific dryness specifications. Finally, medical and pharmaceutical processes may treat water vapor and other gases as contaminants, requiring a very high level of purity.

5. What is the typical range of dew point temperatures to be found in compressed air?

Dew point temperatures in compressed air range from ambient down to -80 °C (-112 °F), sometimes lower in special cases. Compressor systems without air drying capability tend to produce compressed air that is saturated at ambient temperature. Systems with refrigerant dryers pass the compressed air through some sort of cooled heat exchanger, causing water to condense out of the air stream. These systems typically produce air with a dew point no lower than 5 °C (23 °F). Desiccant drying systems absorb water vapor from the air stream and can produce air with a dew point of -40 °C (-40 °F) and drier if required.

6. What are the standards for the quality of compressed air?

ISO8573.1 is an international standard that specifies the quality of compressed air. The standard defines limits for three categories of air quality:

  • Maximum particle size for any remaining particles
  • Maximum allowable dew point temperature
  • Maximum remaining oil content

Each category is given a quality class number between 1 and 6 according to the reference values shown in the table below. As an example, a system that conforms to ISO8573.1 and is rated for class 1.1.1 will provide air with a dew point no higher than -70 °C (-94 °F). All remaining particles in the air will be 0.1 um or smaller, and the maximum oil content will be 0.01 mg/m3. There are other standards for compressed air quality, such as ANSI/ISA- 7.0.01-1996 for instrument air.

table

7. How is dew point in compressed air reliably measured?

Some principles of dew point measurement apply to all types of instruments, regardless of manufacturer:

  • Select an instrument with the correct measuring range: Some instruments are suitable for measuring high dew points, but not low dew points. Similarly, some instruments are suitable for very low dew points but are compromised when exposed to high dew points.
  • Understand the pressure characteristics of the dew point instrument: Some instruments are not suitable for use at process pressure. They can be installed to measure compressed air after it is expanded to atmospheric pressure, but the measured dew point value will have to be corrected if pressure dew point is the desired measurement parameter.
  • Install the sensor correctly: Follow instructions from the manufacturer. Do not install dew point sensors at the end of stubs or other “dead end” pieces of pipe where there is no airflow.

 

In addition to the general principles given above, consider the following when selecting and installing a dew point instrument:

  • The best installation for a dew point sensor isolates the sensor from the compressed air line. This is accomplished by installing the sensor in a “sample cell” and connecting the cell to a “T” in the compressed air line at the point of interest. A small amount of compressed air is then bled past the sensor. The cell should be made of stainless steel and connected to the “T” with tubing (1/4” or 6 mm). It is useful to install an isolation valve between the cell and the air line. This enables easy installation and removal of the sensor.
  • A flow-regulating device is necessary to control to airflow past the sensor. The desired flow rate is only 1 slpm (2 scfh). The regulating device can be a leak screw or a valve. To measure pressure dew point, the regulating device is installed downstream of the sensor, so that when the isolation valve is opened, the sensor is at the process pressure. To measure dew point at atmospheric pressure, the regulating device should be installed upstream of the dew point sensor.
  • Do not exceed the recommended flow rate. When measuring pressure dew point, an excessive flow rate will create a local pressure drop at the sensor. Because dew point temperature is pressure sensitive, this will create an error in the measurement.
  • The best tubing material is stainless steel(SS). Nonmetallic tubing can absorb and desorb water vapor, creating a lag in measurement response. If SS tubing is not available, consider using PTFE or other materials that do not absorb water. Avoid the use of clear plastic tubing or yellow rubber tubing.
  • It is possible to reduce installation costs for permanent dew point instruments by installing the sensor directly in the compressed air line. In these cases it is important to choose a location where the sensor has adequate airflow and where the temperature of the compressed air is at or near ambient.

8. What are the telltale signs of a malfunctioning dew point sensor?

  • An instrument that displays one value all of the time, as if the output or display were locked.
  • An instrument that is “bottomed out,” always reading its lowest possible value.
  • An instrument that is erratic, changing rapidly or randomly over a wide range of values.
  • An instrument that displays impossibly dry or wet dew point values.

9. How often should a dew point sensor be checked or calibrated?

It is best to follow the manufacturer’s recommendation. A one or two year calibration interval is suggested, depending on the instrument. Sometimes a simple field check against a calibrated portable instrument is sufficient to verify correct operation of other instruments. Any time that you have doubts about the performance of your dew point instruments, it is wise to check their calibration.

Visit www.vaisala.com/compressedair for application notes, infographics, animations and product information related to dew point measurement in compressed air.

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